Ontogeny of platelet serotonin (5-hydroxytryptamine, 5-HT) during the first year of life was examined in newborns and infants. The effects of in utero exposure to selective serotonin reuptake inhibitors (SSRI, including fluoxetine, sertraline, and citalopram) were examined by comparing cord blood 5-HT levels in exposed and unexposed newborns. Heritability was assessed by correlation of the platelet 5-HT values observed for mother-infant pairs. No age effect was observed in 1-49 wk-old infants (r ϭ 0.13, p ϭ 0.49) and mean platelet 5-HT levels in infants (241 Ϯ 102 ng/mL, n ϭ 33; 615 Ϯ 320 ng/10 9 platelets, n ϭ 32) were similar to those reported for older children and adults. However, significantly lower blood 5-HT levels were observed in newborns (81.3 Ϯ 32.5 ng/mL, n ϭ 16, p Ͻ 0.0001; 297 Ϯ 101 ng/10 9 platelets, n ϭ 11, p ϭ 0.0007) compared with the 1-49 wk-old infants. The mean cord blood 5-HT concentrations in newborns exposed in utero to SSRI (n ϭ 8) were substantially lower than that seen in unexposed (n ϭ 16) newborns (20.6 Ϯ 14.4 versus. 81.3 Ϯ 32.5 ng/mL, p ϭ 0.0001; 90.7 Ϯ 55.4 versus. 297 Ϯ 101 ng/10 9 platelets, p ϭ 0.0005). Platelet serotonin levels (ng/10 9 platelets) in mother-child pairs (n ϭ 32) were significantly correlated (r ϭ 0.415, p ϭ 0.018). The results indicate that, although platelet 5-HT is low at birth, values quickly increase and stabilize at near-adult levels by 1 mo of age. Gestational exposure to SSRI appears to substantially reduce platelet 5-HT uptake in the fetus, strongly suggesting that such exposure has important physiologic effects. The observed mother-infant correlation agrees with a previous report of high heritability in a large adult population. There is a continuing interest in platelet serotonin (5-HT) due to its hemostatic, thrombogenic, and cardiovascular effects (1, 2), its utility in assessing the bioeffects of SSRI (3-5), and its alteration in several neuropsychiatric disorders (6 -8). In prior work (9 -11), we carried out a series of studies examining the effects of SSRI exposure on platelet 5-HT in nursing infants and in the present study seek to extend the available data regarding the normal ontogeny of platelet 5-HT in infants. We also wished to establish more firmly prior reports of low platelet 5-HT levels in newborn cord blood and to examine the effects of gestational SSRI exposure on cord blood levels.Finally, our investigations of nursing mother-infant pairs provided the opportunity to contribute to the limited data assessing the heritability of platelet 5-HT.The platelet and neuronal 5-HT transporters are encoded by the same gene and appear identical in terms of pharmacology (12, 13). All platelet 5-HT is accumulated by uptake through the platelet membrane 5-HT transporter and a number of studies have used the decline in platelet 5-HT seen after administration of reuptake inhibitors as an index of central and peripheral transporter blockade (3-5). The impact of central serotonin 5-HT modulation by SSRI during critical periods of brain development is un...
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